This invention relates to exhaust gas sensors, and, more particularly, to an oxygen sensor design that incorporates a ground isolation coating over a poison resistance coating.
Gas sensors are used to sense the presence of constituents of exhaust gases, and are typically used in a variety of applications that require qualitative as well as quantitative analysis of gases. In automotive applications, the direct relationship between the oxygen concentration in an exhaust gas and the air-to-fuel ratio of the fuel mixture supplied to the engine allows the gas sensor to provide oxygen concentration measurements for the determination of optimum combustion conditions, maximization of fuel economy, and management of exhaust emissions.
A conventional stoichiometric gas sensor typically consists of an ionically conductive solid electrolyte material, a porous electrode on the exterior of the sensor having a porous protective overcoat exposed to the exhaust gases, and a porous electrode on the interior surface of the sensor exposed to a known oxygen partial pressure. Sensors typically used in automotive applications use a yttria-stabilized zirconia-based electrochemical galvanic cell with porous platinum catalytic electrodes, operating in potentiometric mode, to detect the relative amounts of oxygen present in the exhaust generated by the automobile engine. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:   E  =            (                        -          RT                          4          ⁢                      xe2x80x83                    ⁢          F                    )        ⁢          ln      ⁢              (                              P                          O              2                        ref                                P                          O              2                                      )            
wherein:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=aboslute temperature of the gas
PO2ref=oxygen partial pressurre of the reference gas
PO2=oxygen partial pressureof the exhaust gas
Due to the large difference in oxygen partial pressure between fuel-rich and fuel-lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric gas sensors indicate qualitatively whether the engine is operating fuel-rich or fuel-lean, without quantifying the actual air-to-fuel ratio of the exhaust mixture.
In general, electrodes are constructed around a ceramic electrolyte, which conducts ionic oxygen. The electrolyte develops a voltage when the oxygen concentration varies on opposing sides of the electrolyte surfaces. To measure the oxygen concentration of the exhaust gas, one side of the electrolyte is exposed to the exhaust gas while the other side is kept in contact with air. The voltage across the electrolyte is a function of the difference in oxygen concentration.
The electrodes are typically protected from contamination and erosion as a result of direct exposure to exhaust gas by a single porous poison resistance coating. This poison resistance coating is customarily magnesium aluminate spinel that is flame-spayed, plasma-sprayed, co-sintered, or thermally deposited on top of the exhaust sensing electrode over the active area of the electrode. Generally, the materials and porosity are selected to prevent metal impurities, such as silicon (Si), lead (Pb), calcium (Ca), phosphorus (P), magnesium (Mg), iron (Fe), and zinc (Zn), from permeating the layer and interfering with the operation of the sensor.
Additional coatings are used to isolate the sensor electrical circuit from the vehicle by limiting current flow between the sensor element and shell in the event that the vehicle exhaust system and the sensor are at different electrical potentials. In the prior art, this isolation coating is typically a magnesium aluminate spinel that is plasma-sprayed, co-sintered, or thermally deposited over the electrode where it would contact the metallic sensor package thus preventing a electrical path between the sensor and vehicle exhaust system. Additional prior art has used non-conductive glass coatings. During operation, this layer prevents the flow of current to the electrode as long as the voltage difference between the sensor and vehicle exhaust system is below the dielectric strength value that defines what the coating is capable of withstanding. On the other hand, if the voltage difference exceeds the dielectric strength of the coating, the sensor will experience the increased flow of current. Upon experiencing this increased flow of current, the sensor output voltage can be offset which will be interpreted by the engine management system as incorrect readings of the air/fuel ratio.
There remains a need in the art to have improved ground isolation properties without additional manufacturing operations or additional components.
A sensor element for an exhaust gas sensing apparatus that incorporates an alumina coating is disclosed herein. The gas sensor comprises: an electrolyte having a tip and a protrusion; an inner electrode disposed on an inner surface of said electrolyte body; an outer electrode disposed on an outer surface of said electrolyte body from said tip toward said protrusion; a protective coating disposed over said outer electrode; and an alumina coating disposed over at least a portion of said protective coating.
The method of constructing the gas sensor comprises: forming an electrolyte body having a tip and a protrusion; applying an electrode ink to an inner surface of said electrolyte body to form an inner electrode; applying said electrode ink to an outer surface of said electrolyte body to form an outer electrode from said tip toward said protrusion; depositing a protective coating over said outer electrode; and depositing an alumina coating over at least a portion of said protective coating.